TRANSPORTATION ENGINEERING II CONTACT AREA (TYRE AND ROAD)

The shape of the contact area between tyre and the road is approximately circular when the load is small relative to the maximum recommended load for the tyre, but it gets elongated as the wheel load is increased at a constant inflation pressure.▀

In calculating pavement stresses resulting from the passage of traffic, it is normally assumed that the load carried by the wheel is uniformly distributed over a circular area. The radius of the loading is calculated from the wheel load and the tyre pressure.

The relationship between contact area and the wheel load obtained in this way is shown for the tyre and the loading conditions.▀ The curves for the actual contact area and the computed area are similar for the lowest two wheel loads used when the contact envelope is approximately circular, but they diverge considerably for the higher wheel loads.

Measurements of actual pressure acting between the tyre and the road shows that, where the load ratio (actual wheel load : recommended maximum wheel load) for the tyre is small, the distribution of pressure across the tyre area is parabolic. Most heavily loaded wheels are fitted with dual tyres, so spaced that at the correct tyre pressure the deformed parts of the tyres in contact with the road do not touch each other.